A British peace activist taken hostage in Iraq was shown in a video broadcast on Arab television today, in which a previously unknown insurgent group claimed responsibility for the kidnapping.

Professor Norman Kember was shown sitting on the floor next to three other Western men, believed to be the four who were captured in the Iraqi capital on Saturday.

The father of two was taken captive with two Canadians and an American as he left a mosque in Baghdad where he had been talking to local leaders about human rights abuses.

The Swords of Righteousness Brigade said the four were spies working undercover as Christian peace activists, al-Jazeera television news reported. The station said it could not verify any of the information on the tape.

The tape also showed a British passport apparently belonging to Prof Kember. Unlike in some previous videos released by hostage takers in Iraq, he and his colleagues were not caged, and were not made to wear orange overalls like those worn by terror suspects held by the US at Guantanamo Bay.

Prof Kember is a distinguished physicist who helped to develop radiotherapy cancer treatment to save thousands of lives. The 74-year-old pacifist and peace campaigner went to Iraq 10 days ago to investigate reports of human rights abuses and of Muslims being detained illegally.

Friends today dismissed the spying claim. Peace campaigner Bruce Kent told BBC News 24: "He would never ever dream of doing anything like spying. The last thing he would do would be working for the British government."

Pat Gaffney, a friend, told Sky News: "I’m pleased that presumably Norman and the other folk are still alive and that hopefully in good health. But still very concerned about the danger and particularly now the interpretation that’s been given that they’re spies. We’ve really got to do all we can to try and dispel that myth."

The Foreign Office issued a brief statement tonight denouncing the kidnapping. It said: "We utterly condemn the abduction of Norman Kember and his colleagues. The release of this video can only cause further distress to their families at this difficult time."

The Iraqi government has pledged "every assistance" in the effort to find the hostages.

Paul Bigley, brother of murdered British hostage Ken Bigley, said the Kember family were welcome to speak to him if they needed to do so.

"If the Kember family think they would like to speak to me, and maybe share some of the things I did and some of the numbers and some of the people I spoke to, they’re more than welcome, if they so wish," he told the Live with Alastair Stewart Show on the ITV News Channel.

Prof Kember, born in London, graduated in physics before joining Professor Len Lamerton at the Institute of Cancer Research in the late 1950s. The institute became the Royal Marsden Hospital in London. He went on to work at the Royal Free Hospital’s Department of Medical Physics and at St Bartholomew’s Hospital in London. His work largely focused on establishing what safe doses of radiotherapy could be given to cancer victims.

His wife Patricia, 72, made no comment as she left their home yesterday in Pinner, northwest London. Prof Kember's family said in a statement before the video was shown: "Norman feels very strongly that the occupation in Iraq is a mistake. He has been a pacifist all his life, working in hospitals rather than doing national service at the age of 18."

Chris Cole, director of the Fellowship of Reconciliation, said: "He has consistently opposed war and violence. He has worked for peace and to educate in particular young people about the power of non-violence. All members and supporters of our organisation are praying for him and his wife, Pat."

Since the invasion of Iraq more than 200 foreigners and thousands of Iraqis have been abducted, most of them held by criminals seeking cash.

In an earlier video relating to a separate kidnapping incident, insurgents today threatened to kill a German woman archaeologist unless her government cuts off ties with the Iraqi government.

German state television said that a video cassette had been delivered to its representatives in Baghdad, showing Susanne Osthoff and her Iraqi driver blindfolded and kneeling on the floor, flanked by three masked men, one armed with a rocket-propelled grenade.

In it, the kidnappers threatened to kill both hostages within a "very short time limit" unless their demands were met. The video was the first news to be heard of the pair since they were abducted on Friday in the historic site of Ninawa in northwestern Iraq.

Osthoff, 43, a Muslim convert, archaeologist and aid worker, fluent in Arabic, had been working in Iraq since before the American-led invasion in 2003. In an interview last month she said she believed she had become a target for kidnapping by the group led by Abu Musab al-Zarqawi after receiving threats while living in Mosul last summer.

She told a German magazine that American soldiers had taken her from Mosul to the heavily fortified Green Zone in Baghdad where they thought she would be safer. But she apparently returned north to pursue her goal of setting up a German cultural centre in the Kurdish north.

The German government today condemned the kidnapping and called for her release without comment on the group’s demands.

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end